Hi again!
I think my problems were solved! 
I followed the suggestion by @eXTric on this thread, and now I think everything is going fine.
Well, I don’t understand why that problems happened just because I blocked SSH connections through IPv6 to SSH (this is part of a little “checklist” I have/do always when I pick a new server), but, who knows.
This time I noticed that the installation process delayed a bit more. And now I was asked to enter the password for the admin user (this never happened before during the several installs I done). The installer also displayed some “DNS updated” messages.
Well, I think that now everything should really be going well, and I was also able to fix the reverse DNS (but MiAB is not “detecting” it yet).
I have some errors in the MiAB stats page (MTA-STS policy is missing: STSFetchResult.NONE, etc), but I am very happy now things start doing good.